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re: was sizing
17 apr 2000
george ghio   wrote:

>...i still think that a 20 hour wait to get the full output from
>my batteries a bit much to put up with...

perhaps not, over a 5-day discharge, but you might prefer 5 w resistors
for 4 hour warming, as already discussed. 

>...cold batteries take longer to charge so they will need to be warmed
>before charging to get the most from the hours of sunlight you have.

pv panels start cold at dawn on a winter day, as do batteries, and cold
panels make more voltage, as required for cold battery charging. later,
the panels warm up in the sun and the batteries warm up because of their
charging, so the panel voltage decreases, along with the battery charging
voltage requirement, so these things tend to balance out naturally.

if we charge a 0 c battery with 1 wh and (a) discharge it at 0 c or
(b) discharge it at 25 c, what's the round trip energy efficiency
in each case, according to actual tests or the manufacturer's specs? 

>warming your batteries to increase their out put is well knowen.
>nick pine says that they will last longer if kept cold.

how much longer, exactly, as a function of temperature?

>cold batteries have reduced output.

their loss of capacity reappears when they are warmed.

>cold batteries take longer to charge.

they require more charging voltage to store the same number of amp-hours.
but how much of this "inefficiency" disappears with subsequent warming? 

>solution warm them up.

strategically, with the help of a smart inverter.

>if you warm them to get more out of them as well as to get more into
>them and you have a 24 hour cycle when do they get cold?

over 8-12 hours, naturally, in an insulated box, or faster if needed,
with active ventilation to the outdoors.

>why not us them at their rating? 

let's try this again, with an example. suppose it's 0 c outdoors for a
month, and your maximum dod is 80%, one time during that month. let's
say our battery stores 200 ah at 6 v, ie 1200 wh, and it weighs 50 lb.
it only takes 20 wh to heat 50 pounds of lead 25 c, but that heating
"releases" 23% more capacity, ie 276 wh, for a net gain of 256 wh. but
there's more: consider these two scenarios:

  1. keep the battery 25 c for the whole month.

     at 25 c, the self-discharge rate is about 12%/week, according to
     fig. 5.9 on page 93 of dr. komp's book, so by the end of the
     month, it's down to 0.88^4 = 60% of capacity, ie it only contains
     0.6x1200 = 720 wh. so it goes dead before the end of the month,
     and it's lost some lifetime as well, compared to scenario

  2. keep the battery 0 c initially, and warm it to 25 c at the end.

     at 0 c, the self-discharge rate is about 2% per week, so by the
     end of the month, it's only down to 0.98^4 = 92% of its capacity,
     and removing another 20 wh to heat it to 25 c makes this 1087 wh,
     ie 90.5% of the initial charge...

in scenario 2, the battery stores 1087/720 = 51% more energy, no?

and in scenario 2, the battery lasts longer.

>if they are used outside of their rating will the manufacturer pay the
>warranty if they fail?

check your warranty. many derate lifetimes at higher temperatures.


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